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Dual string configuration

  • 14-04-2021 10:08am
    #1
    Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    Just wondering do you HAVE TO have two strings on the one inverter with the same amount of panels and what happens if one string has more?
    Question is can you go 12 panels on one string and 9 panels on the other without any issues? Space constraint is the issue here.
    All panels are the same spec.


Comments

  • Registered Users Posts: 2,683 ✭✭✭ graememk


    SD_DRACULA wrote: »
    Just wondering do you HAVE TO have two strings on the one inverter with the same amount of panels and what happens if one string has more?
    Question is can you go 12 panels on one string and 9 panels on the other without any issues? Space constraint is the issue here.
    All panels are the same spec.

    Each string is independant of each other, so yes you can have 12 on one and 9 on the other (as long as its still within spec of the inverter)


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    I would guess that since the inverter is rated max 8kw DC then each string can get up to 4kws and If I go for 12 on the one string, they add up to 4kw exactly.
    Also on a south facing roof, does it make any difference to generation what orientation you put the panels into? landscape or portrait?


  • Moderators, Category Moderators, Motoring & Transport Moderators, Regional East Moderators Posts: 58,818 CMod ✭✭✭✭ unkel


    SD_DRACULA wrote: »
    I would guess that since the inverter is rated max 8kw DC then each string can get up to 4kws and If I go for 12 on the one string, they add up to 4kw exactly.

    You assume wrong. Each string can have a maximum voltage as per the spec of your inverter. Say this is 800V, that means each string can be up to about 20 panels. It is often quite possible to have an inverter that is rated for say 6kW to have panels of 9kW connected to it no bother (as long as it is within spec of the inverter)

    "Wind is Ireland's oil" - An Taoiseach, 25/05/2022



  • Registered Users Posts: 430 ✭✭ mun1


    I have 12 panels on one string and 4 on the other string.
    All works fine, just different voltages on each string.
    As said above just watch max voltage limit of inverter , which usually equals about 19 panels


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    Hmm that could a problem didn't even think about it.
    Inverter Max Input voltage is 600V (solis 6kw hybrid)
    Panel Voltage (Qcells 335w G9 all black) at peak power is 33.81V so 33.81 * 21 is 710v which is way over (assuming all panels maxing out) but even 18 of them is 610v so does that mean I can't ever max them out? Or no chance of those values ever reaching in Ireland?


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  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    One thing to think of, the inverter is Dual MPPT so it's actually advantageous (I think) to use both strings.

    If you have some panels that are likely to shade worse in winter than others, you could put divide them into two groups and have a 'summer' and 'winter' string.

    The panels that get shaded then won't drag down the unshaded ones. Cheaper than adding optimisers to every panel :)

    On your question around adding 610v to a 600v inverter, not a good idea. If nothing else I suspect the insulation on the cabling might be rated for 600vdc. I'm not sure if the inverter could potentially handle it but why chance it. Much better to have two strings and respect the limits.

    As to being in Ireland, we actually can get good conditions in summer for solar because we get strong sunshine in the summer but with cooler air temperatures we get good performance. Anyway, the panels voltage doesn't change much - it's the current that changes!


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    100%
    Plan is to use either 9 or 12 on the string with the shed panels which get shaded by the house in winter and 9 for the front house roof.
    If I do go for 21 total what happens on a perfect day in case they all max out? It says 4kw per string and my setup would be 4 and 3.


  • Registered Users Posts: 2,683 ✭✭✭ graememk


    SD_DRACULA wrote: »
    Hmm that could a problem didn't even think about it.
    Inverter Max Input voltage is 600V (solis 6kw hybrid)
    Panel Voltage (Qcells 335w G9 all black) at peak power is 33.81V so 33.81 * 21 is 710v which is way over (assuming all panels maxing out) but even 18 of them is 610v so does that mean I can't ever max them out? Or no chance of those values ever reaching in Ireland?

    its 600V per string,

    Was discussing oversizing my array with quentingargan, and you have to allow for temperature gradient
    We were talking about the longi panels which are about 40v per panel.

    Linky : https://www.boards.ie/vbulletin/showpost.php?p=116585187&postcount=32

    Max I could put on each string is 12.

    I am noticing some clipping at the top on the days with scattered cloud, Ie maxing out the 6kw of the inverter, but today we had steady sunshine and was topping out at about 5.8ish kW


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    Yes I am debating whether to throw in 3 extra panels as I can just about fit them on the shed (1 extra kw for a total of 7)
    And yes the inverter is max 8kw DC and 6kw AC so does it mean that in case the panels are reaching 7 kws it can push 6 kws to the house/grid and 1kw to the battery?


  • Moderators, Sports Moderators Posts: 15,993 Mod ✭✭✭✭ slave1


    Surely if clipping DC generation the inverter will simply limit to its maximum input?
    That also means no Count, 6kW in total, never 7kW because as above the inverter limits DC input.
    Maybe I’m wrong but pretty sure that’s how it works


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  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    slave1 wrote: »
    Surely if clipping DC generation the inverter will simply limit to its maximum input?
    That also means no Count, 6kW in total, never 7kW because as above the inverter limits DC input.
    Maybe I’m wrong but pretty sure that’s how it works

    Not that simple, the inverter connectors and internals probably have 600vdc max. Insulation and arc distance etc. If inverter specifies Max 600vdc I’d not like to try and see...


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    Hmm so 21 of these https://midsummer.ie/buy/qcell-solar/Qcell-335-G9-BLK will be too much for the inverter? Surely that's only the case if they run at the max values and real life it will be lower?


  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    SD_DRACULA wrote: »
    Hmm so 21 of these https://midsummer.ie/buy/qcell-solar/Qcell-335-G9-BLK will be too much for the inverter? Surely that's only the case if they run at the max values and real life it will be lower?

    As explained previously. Panels are current variable for the most part NOT voltage. Strongly advise you don’t exceed nominal maximums.


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    In that case should be good as inverter is max 8kws DC and I am putting 7kws split in two strings.


  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    SD_DRACULA wrote: »
    In that case should be good as inverter is max 8kws DC and I am putting 7kws split in two strings.

    You keep mixing up units. Max per string VOLTAGE is 600vdc (on Solis 5kw hybrid, obviously you need to look at your specs).

    Maximum AC output is 5kw irrespective of which / how many strings.

    In other words, you can have 2x 5kw strings attached and at maximum production and the unit will still only output 5kw.

    You need to separate in your head the Max DC voltage input from the solar panels and the maximum AC power ( which is volts * Amps) the inverter can supply from that DC input.


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    Yes I am aware of the max 6kws AC limit on the inverter which is fine (and regulation is also 6kw max for single phase in Ireland anyway)
    Question was, is there an issue with this many panels on the same inverter but my installer confirmed that voltage doesn't really matter as much as current so should be good.


  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    SD_DRACULA wrote: »
    Yes I am aware of the max 6kws AC limit on the inverter which is fine (and regulation is also 6kw max for single phase in Ireland anyway)
    Question was, is there an issue with this many panels on the same inverter but my installer confirmed that voltage doesn't really matter as much as current so should be good.

    Ok..... if installer said that big red alarm bells should be going off.


  • Registered Users Posts: 2,683 ✭✭✭ graememk


    SD_DRACULA wrote: »
    Yes I am aware of the max 6kws AC limit on the inverter which is fine (and regulation is also 6kw max for single phase in Ireland anyway)
    Question was, is there an issue with this many panels on the same inverter but my installer confirmed that voltage doesn't really matter as much as current so should be good.


    I have 24 panels on my solis 6kw inverter.

    12 on each string. - the calculations for my setup:

    You can push it to 24 panels - 12 per string, but no more than that. Open circuit voltage is 40.8.

    40.8x13x1.15 is 610V. Too high.

    Doing it the hard way, for that panel type, temperature coefficient is 0.27% per degree. You have to allow 45 degrees between test conditions of 25c and lowest air temperature of -20c. Tolerance on Voc is 3%.

    Worst scenario at -20 degrees is 40.8 x 13panel x 1.1215 temp coeff x 1.03 tolerance = 612V

    The voltage is fairly steady - it is mostly current that is affected by light intensity.


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    Well if your house did not explode with 12 and 12 then I am quite confident mine will not either with 9 and 12 :D


  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    SD_DRACULA wrote: »
    Well if your house did not explode with 12 and 12 then I am quite confident mine will not either with 9 and 12 :D

    12 panels is well less than enough to overload a string. Your posts are confusing as you keep changing the question.

    You originally asked about putting 21 panels up "Panel Voltage (Qcells 335w G9 all black) at peak power is 33.81V so 33.81 * 21 is 710v which is way over (assuming all panels maxing out) but even 18 of them is 610v so does that mean I can't ever max them out? Or no chance of those values ever reaching in Ireland?"

    I tried to explain that you can't put 21 panels on one string as it would overload the inverter but split between 2 strings would be fine.

    You then asked "100% Plan is to use either 9 or 12 on the string with the shed panels which get shaded by the house in winter and 9 for the front house roof.
    If I do go for 21 total what happens on a perfect day in case they all max out? It says 4kw per string and my setup would be 4 and 3."

    Then you went back to talking about voltage again. "Hmm so 21 of these https://midsummer.ie/buy/qcell-solar/Qcell-335-G9-BLK will be too much for the inverter? Surely that's only the case if they run at the max values and real life it will be lower?"

    And again, I tried to explain that panels are current variable but not voltage.

    Then you switched again to talking about power "In that case should be good as inverter is max 8kws DC and I am putting 7kws split in two strings."

    Then you posted this " Yes I am aware of the max 6kws AC limit on the inverter which is fine (and regulation is also 6kw max for single phase in Ireland anyway)
    Question was, is there an issue with this many panels on the same inverter but my installer confirmed that voltage doesn't really matter as much as current so should be good."

    Which is totally incorrect, of course voltage matters. Inverter has max 600vdc per string limit.

    Hopefully the only person who isn't confused at this stage is you as it's your project :) Or maybe I'm just a bit dim and I'm confusing you more than I'm helping :(


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  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    I think I'll leave it to the expert in charge of fitting the system.
    I'll pass on all the stuff that we discussed here and let him make the call as long as it is safe to do so.
    Original plan was to split it by two strings of 9 (don't see an issue with that) and then I realized I could add 3 more to the shed string - that was really the question, whether it would be safe to go with a 9 and 12 string combo.


  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    SD_DRACULA wrote: »
    I think I'll leave it to the expert in charge of fitting the system.
    I'll pass on all the stuff that we discussed here and let him make the call as long as it is safe to do so.
    Original plan was to split it by two strings of 9 (don't see an issue with that) and then I realized I could add 3 more to the shed string - that was really the question, whether it would be safe to go with a 9 and 12 string combo.


    Ok - summary:

    The answer is yes. Assuming Solis Hybrid I'm familiar with:

    You can fit 2x strings of Panels.

    Each string can have a maximum voltage of 600vdc. So with those G9 panels you could have 14 of them on each string. (40.87v each panel/600=14.68). So theoretically you could have 28 panels total.

    That would give you a theoretical max of 4.7kw on each string. But that doesn't give you any overhead so I'd stick with 12 max.

    But as the inverter is 5kw, the most you could output on the AC side is ~5kw between both strings. (i.e the max is 5kw total output power across both strings).


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    Yeah that's it except there is a newer 6kw solis hybrid inverter just out so I'm going for that because why not.


  • Registered Users Posts: 1,268 ✭✭✭ SD_DRACULA


    To put this to rest, straight from the Solis designer
    All is green :D


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